1. Field of the Invention
This invention relates to an auto-tracking control system suitable for use with a video tape recorder having a rotary head drum assembly, and more particularly to an auto-tracking control system in which a magnetic head is mechanically vibrated transverse to its scanning path at a relatively low frequency to correct tracking error and is dithered at a high frequency across its scanning path to generate a tracking control signal.
2. Description of the Prior Art
The prior art contains examples of systems in which a reproducing magnetic head in a helical scan video tape recorder for reproducing video signals recorded on a magnetic tape is mounted on a piezo-electric element which is deflected in the width direction of a recorded track by a control signal to control the scanning path of the reproducing magnetic head with respect to the recorded track. Such piezo-electric elements conventionally include bi-morph leaves of piezo-ceramic material which respond to control voltages applied thereto.
A bi-morph leaf includes two plates of piezo-ceramic material longitudinally aligned with each other and bonded together at their abutting faces. Electrodes deposited on both surfaces of each plate permit the application thereto of control signals. In response to control signals the plates deflect together with an amplitude and direction normal to their abutting faces which is determined by the magnitude and polarity of the control signals applied thereto.
One type of control signal applied to bi-morph leaves of the prior art is a relatively high frequency "dithering" or "wobbling" signal which rapidly deflects the reproducing magnetic head in small-amplitude sine-wave excursions in the width direction about a mean scanning path. The reproduced signal from the reproducing magnetic head contains amplitude variations as the reproducing magnetic head is dithered across the recorded track successively into and out of coincidence therewith. The phase of the amplitude variations is compared with the phase of the dithering or wobbling control signal to derive from the comparison a tracking control signal which is then used to align the mean scanning path of the reproducing magnetic head with the recorded track.
Bi-morph leaves can be made with a resonant frequency of as high as about 900 Hz. The dithering or wobbling frequency is desirably high but is limited to a value below this frequency. A frequency of about 720 Hz has been conveniently employed in auto-tracking control systems of the prior art.
A second type of control signal is required especially when a tape transport speed is used during reproduction which is different from the tape transport speed used during recording. Skewed parallel recorded tracks in a helical scan system have a skew angle which is partly established by the tape transport speed employed during recording. When reproduction is performed at still, slow or fast tape transport speeds, the head scanning path is skewed with respect to the recorded track. Conventionally, each recorded track contains video information for all of the television lines in a field or a frame, depending on the system. One recorded track is scanned in 1/60 second (one field per track) or 1/30 second (one frame per track). To correct for skew of a recorded track with respect to a scanning path, a saw wave control signal applied to a bi-morph leaf is smoothly varied during the time the reproducing magnetic head travels from one end of a recorded track to the other. Thus, the saw wave control signal has a repetition frequency of from about 30 to about 60 Hz. Besides being of lower frequency than the dithering displacement, the amplitude of the low-frequency deflection of the reproducing magnetic head required to compensate for track skew is much larger. A bi-morph leaf for low frequency, large amplitude response is designed with some combination of material, shape, greater mass, width, length and thickness to accomplish this response. A bi-morph leaf for high frequency, small amplitude response is designed with some combination of material, shape, smaller mass, width, length and thickness. Thus the parameters required to accomplish high and low frequency response are mutually exclusive. An attempt to provide a single compromise bi-morph leaf capable of responding to both high and low frequency control signals results in a device having inferior response to both.
In U.S. Pat. No. 4,099,211 issued July 4, 1978, a single composite control signal containing the above-described high and low frequencies is supplied to a bi-morph leaf assembly which includes first and second bi-morph leaf elements connected to each other in longitudinal alignment and having similar frequency characteristics. Direct and inverted phases of the composite control signal are applied to electrodes of the first and second leaf element, respectively, to deflect the bi-morph leaf element in opposite directions. Therefore, the gap of a reproducing magnetic head mounted on one of the leaf elements is deflected to dither as previously described and to maintain the scanning path traced by the gap of the reproducing magnetic head appropriately aligned with the recorded track on the tape surface during auto-tracking. Moreover, due to the opposite deflections of the first and second leaf elements, the contact surface of the reproducing head is not angled relative to the tape surface as a consequence of the deflections of the head.
As described above, the high and low frequencies required for dithering and for correction of track skew are related by frequency ratios of 12 or 24. This wide frequency difference makes it difficult or impossible to equally accommodate both signals with a single bi-morph leaf. In addition, the displacement required of the magnetic head in response to the low frequency component of the control signal is much greater than the displacement required in response to the high frequency component. The differing amplitude requirements make it even more difficult to produce a single bi-morph leaf capable of satisfying the conflicting requirements. Consequently, satisfactory auto-tracking control operation is not obtainable with an electro-mechanical transducing element or elements designed for either high frequency-low amplitude operation or low frequency-high amplitude operation, or for a compromise therebetween.